1. Field of the Invention
The present invention relates to a method for manufacturing an optical element which is used for an exposure apparatus, a photo-taking apparatus, an illumination apparatus, etc. The optical element includes, for example, a diffractive optical element, a spherical lens, an aspherical lens, etc.
2. Description of Related Art
Diffraction gratings have heretofore been used as spectral elements for spectroscopes. Some of the diffraction gratings are in a saw-tooth-like sectional shape called a blazed type, and the diffraction efficiency of some of the diffraction gratings reaches 100%. Meanwhile, as optical elements which also utilize diffraction, binary optics (abbreviated as xe2x80x9cBOxe2x80x9d) elements, which have diffraction gratings formed in a step-like sectional shape, have recently come to draw attention. A BO lens, which is one of the BO elements, is expected to give achromatic effect and aspherical effect and is, therefore, expected to greatly contribute to the development of novel optical systems using ultraviolet rays, etc.
In a case where the BO lens is to be used for a stepper (projection exposure apparatus), for example, the BO lens is required to meet specifications which much exceed the current machinable limit for the blazed-type diffraction grating. However, fine machining on a diffraction grating can be carried out with a certain degree of precision by a lithographic process employed in machining a semiconductor wafer. For example, a BO lens having eight steps aligned thereon can be manufactured by a photo-lithography by using quartz for the substrate of the BO lens, using an i-ray stepper (semiconductor exposure apparatus) for an exposure printing process, and using a parallel-plate-type reactive ion etching (RIE) apparatus for a dry etching process.
In manufacturing the BO lens in such a manner, if the BO lens is desired to be about 20 mm in diameter, for example, the BO lens can be manufactured by repeatedly performing an exposure process a total of three times by using three masks patternized with chromium, and performing a developing process and a dry-etching process respectively a total of three times. In actually manufacturing a large BO lens which measures 200 mm or thereabout in diameter for the lens used for the stepper, however, it is necessary to make divisional exposures which are almost one hundred in number, and at least fifteen chromium masks are required for the one hundred divisional exposures.
The conventional manufacturing method as described above thus necessitates many manufacturing processes. The large number of processes lowers productivity and increases the cost of manufacture. To solve this problem, it is conceivable to manufacture the BO lens by a molding method. However, in the case of a BO lens to be included in an optical system adapted for ultraviolet rays and particularly for far ultraviolet rays, the material usable for the BO lens must be selected from a limited range of high-melting-point materials such as quartz, fluorite, sapphire, etc. Hence, it is very difficult to find any metal material that is usable as a mold (die) and is capable of withstanding such a high temperature at which the high-melting-point material is to be melted for molding. Besides, in such a case, the precision of machining would be deteriorated by the thermal expansion of the mold material and the contraction of the material taking place when the material cools down. The problem of contraction taking place when the material cools down also takes place in a case where a BO lens or aspherical lens for visible rays is to be manufactured by molding with a glass or plastic material.
Meanwhile, machining for an aspherical surface of a lens or the like is impossible by a lapping process employed as one of methods for machining work on the spherical and level surfaces of a spherical lens, a prism, etc.
In machining an aspherical surface, it has been practiced to adopt, for example, a stripping machining process whereby the aspherical surface is machined with loose abrasive by pressing a finishing shape against the surface. It is a basic concept of the stripping machining process to obtain a desired shape by generating a working amount distribution. However, according to the stripping machining process, not only work efficiency is low, requiring much time and labor for measuring and repeating machining work on necessary parts, but also some skill in machining work is necessary.
Known aspherical surface processing methods include an adding machining process. The adding machining process is carried out to obtain an optical aspherical surface by vapor-depositing, on a polished glass surface, aluminum for a reflecting mirror, or ZnS for a light-transmissive element, and varying the thickness of the vapor-deposited film through a vapor deposition mask. It is also known to obtain an aspherical surface by controlling an attaching amount of metal plating.
A deforming-work molding process is also known as another aspherical surface processing means. According to the deforming-work molding process, with both a material and a mold being heated, an optical aspherical surface is obtained by deforming the material while pressing the material against the mold in the heated state. The force of deformation can be generated by its own weight of the material, or by applying pressure or by pulling the material under reduced pressure.
However, the stripping machining process is not suited for mass production as it has a low work efficiency and requires some skill for the work.
In the case of the adding machining process, the controllable limit of thickness of the vapor-deposited film is several xcexcm. Therefore, both the plating method and the vapor deposition method are hardly suited for obtaining an aspherical surface at a high rate of precision. Besides, the plating method is usable only for forming a reflecting optical surface.
Further, the deforming-work molding process not only presents problems in respect of mold releasability, surface denaturation, etc., but also necessitates heating the material up to a temperature higher than a transition point of the material for the purpose of deforming the material as desired. Even in a case where a low melting-point glass material is employed, the glass material must be heated up to a temperature between 320xc2x0 C. and 330xc2x0 C. The usable wavelengths for a lens obtained by using a low melting-point glass or plastic material to which the deforming-work molding process is applicable are visible wavelengths. In order to obtain a lens which is usable for the wavelengths of ultraviolet rays, particularly, far ultraviolet rays, a high melting-point material such as quartz, fluorite, sapphire or the like must be used.
Therefore, the material of a mold to be used must be selected by considering various properties resulting from a thermal reaction, including the fusibility of a metal mold, mold releasability, oxidation resisting property, the characteristics of denaturation and deterioration of the surface, and the thermal expansion, heat conduction, high temperature strength, grain boundaries, porosity, etc., required for retaining geometric precision and surface roughness. In addition to these matters, the basic conditions required for the preparation of a mold including the workability, machinability, thin-film adhesiveness, cost, etc., of the mold must be taken into consideration. The range of selectable materials is, therefore, extremely narrow.
It is an object of the invention to provide an improved method for manufacture of an optical element, by which the above-stated problems of the prior art can be solved.
To attain the above object, in accordance with an aspect of the invention, there is provided a method for manufacture of an optical element, the method comprising a step of forming a thin film on a surface, such as a grating surface, an aspherical surface or a spherical surface, formed on a mold, a step of bonding a substrate to the thin film, and a step of separating the thin film and the substrate from the mold.
Further, in accordance with another aspect of the invention, there is provided a method for manufacture of an optical element, the method comprising a step of forming a thin film on a surface, such as a grating surface, an aspherical surface or a spherical surface, formed on a mold, a step of polishing a surface of the thin film, a step of bonding a substrate to the polished surface of the thin film, and a step of separating the thin film and the substrate from the mold.
The above and other objects and features of the invention will become apparent from the following detailed description of preferred embodiments thereof taken in connection with the accompanying drawings.